Understanding the structure and properties of geological formations can improve the efficiency of oil field operations such as drilling, well completion, and production. The collection of information relating to conditions downhole, commonly referred to as “logging,” can be performed by several methods including nuclear magnetic resonance (NMR) logging.
NMR logging tools operate by using an imposed static magnetic field, B0, to give nuclei with non-zero nuclear spin (non-zero angular momentum) split energy levels. Since lower energy levels are preferred, an ensemble of nuclei will exhibit an anisotropic distribution of energy states, giving the nuclear spins a preferential polarization parallel to the imposed field. This state creates a net magnetic moment, producing a bulk magnetization. The nuclei (primarily hydrogen nuclei) converge upon their equilibrium alignment with a characteristic exponential relaxation time constant. When this convergence occurs after the nuclei have been placed in a cooperative initial state (discussed below), it is known as recovery. The time constant for recovery is called the “spin-lattice” or “longitudinal” relaxation time T1.
During or after the polarization period, the tool applies a perturbing field, usually in the form of a radio frequency electromagnetic pulse whose magnetic component, B1, is perpendicular to the static field, B0. This perturbing field moves the orientation of the magnetization into the transverse (perpendicular) plane. The frequency of the pulse can be chosen to target specific nuclei (e.g., hydrogen). The polarized nuclei are perturbed simultaneously and, when the perturbation ends, they precess around the static magnetic field gradually re-polarizing to align with the static field once again while losing coherence in the transverse plane (T2 relaxation). The precessing nuclei generate a detectable radio frequency signal that can be used to measure statistical distributions of T1, T2, porosities, and/or diffusion constants.
Due to B0 in an ex-situ NMR logging tool being a gradient field there is a range of Larmor frequencies available. Thus, depending on the magnetic field configurations being used in an NMR experiment, controllable B1 parameters (e.g., amplitude, frequency, pulse bandwidth) can improve the signal-to-noise ratio (SNR) of NMR logging measurements. Determining appropriate control parameters for NMR logging operations is not a trivial task. Further, temperature variance, or other dynamic variables, in the downhole environment may necessitate changes to NMR logging control parameters.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.